Method And System For Real-Time In-Process Measurement Of Coating Thickness

ABSTRACT

The present disclosure is generally directed to methods and systems for measuring the thickness of coatings or thin films on various substrates. For example, one disclosed method includes the steps of providing and directing light waves of varying wavelengths toward a moving substrate comprising a coating, linearly polarizing the light waves, converting the linearly polarized light waves to circularly polarized light waves, analyzing elliptically polarized light waves reflected by the moving substrate, capturing analyzed light waves, generating light wave data based on the captured light waves, and determining a thickness of the coating based on the light wave data.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/781,457 filed Sep. 30, 2015, entitled “Method & System For Real-TimeIn-Process Measurement Of Coating Thickness,” which claims priority toPCT Application No. PCT/US2014/027980 filed Mar. 14, 2014, entitled“Method and System For Real-Time In-Process Measurement of CoatingThickness,” published as WO 2014/143838, which claims priority to U.S.Provisional Patent Application No. 61/792,689 filed Mar. 15, 2013,entitled “Method and System for Inline Real-Time Measurement of ThinFilm Thickness.” The content of each of the aforementioned applicationsis incorporated by reference herein in its entirety.

FIELD

The present disclosure generally relates to methods and systems formeasuring the thickness of coatings or thin films on various substrates.Embodiments include methods and systems for real-time in-processmeasurement of coating thickness, and more particularly methods andsystems for real-time in-process measurement of a coating thickness on amoving substrate or a coated product.

BACKGROUND

The measurement of thin films and coatings is appreciated inmanufacturing settings. For example, regulating the application of athin film or coating to a product within a preferred thickness rangeallows manufacturers to ensure that a film or coating is applied withsufficient thickness to prevent manufacturing defects while alsoavoiding wasteful application of film or coating in excess of a requiredthickness, thereby minimizing materials costs. In the context ofmanufacturing processes, measurement of thin layers of lubriciouscoatings on metals allows manufacturers to ensure that sufficientcoating is applied to prevent substantial damage to expensivemanufacturing and processing equipment. Furthermore, it is preferable toperiodically perform measurements of thin films or coatings in real-timeas the films or coatings are applied before the coated substrateproceeds further through the manufacturing process.

There are several currently known techniques for measuring thin films orcoatings. However, the known methods have various limitations thatsignificantly undermine their respective usefulness in industrialapplications. For example, standard reflectometry based measurementtechniques become unreliable when the thickness of the subjectcoating/film under consideration is below 200 nanometers (1nanometer=0.001 microns). Known modeling-based reflectometry techniquesare not well-suited and not robust enough for use in industrialproduction environments.

In addition, modeling-based reflectometry techniques for measuring filmor coating thicknesses of less than 0.2 microns have typically focusedon measurement of coatings and films on semiconductor substrates.However, applications of coatings or films in the semiconductormanufacturing process are performed on static (non-moving) substrates. Asignificant limitation of the currently known modeling-basedreflectometry techniques capable of measuring coatings of less than 0.2microns is that they require a static substrate on which to performmeasurements of coatings or materials deposited thereon.

The present disclosure is directed to methods and systems for real-timein-process measurement of thin films or coatings, including films orcoatings of less than 0.2 microns, on a moving substrate, and thereforeovercomes the limitations of known methods and systems.

SUMMARY

The present disclosure generally relates to a method comprisingproviding and directing light waves of varying wavelengths toward amoving substrate comprising a coating, linearly polarizing the lightwaves, converting the linearly polarized light waves to circularlypolarized light waves, analyzing elliptically polarized light wavesreflected by the moving substrate, capturing analyzed light waves,generating light wave data based on the captured light waves, anddetermining a thickness of the coating based on the light wave data. Inanother embodiment, the method further comprises detecting a disturbancein the movement of the substrate based on the light wave data andadjusting an orientation of an analyzer and or an orientation of adetector based on the detected disturbance.

The present disclosure is also generally related to a system comprisinga processor, a light source in communication with the processor, thelight source configured to provide and direct light waves of varyingwavelengths toward a moving substrate comprising a coating, a polarizerpositioned between the light source and the moving substrate, a waveplate positioned between the polarizer and the moving substrate, ananalyzer positioned to receive light waves reflected by the movingsubstrate, a detector, in communication with the processor, positionedto capture light waves reflected by the analyzer and configured togenerate light wave data based on the captured light waves, and a memoryin communication with the processor, wherein the memory comprisescomputer program code executable by the processor to determine athickness of the coating based on the light wave data. In anotherembodiment, an orientation of the analyzer is adjustable and anorientation of the detector is adjustable. In still another embodiment,the memory further comprises computer program code executable by theprocessor to: detect a disturbance in the movement of the movingsubstrate based on the light wave data; and adjust the orientation ofthe analyzer or the orientation of the detector based on the detecteddisturbance.

Illustrative embodiments disclosed herein are mentioned not to limit ordefine the invention, but to provide examples to aid understandingthereof. Illustrative embodiments are discussed in the DetailedDescription and further description of the invention is providedtherein. Advantages offered by various embodiments of this invention maybe further understood by examining this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages according to thepresent disclosure are better understood when the following DetailedDescription is read with reference to the accompanying figures, wherein:

FIG. 1A is a block diagram of a system for measuring thin film/coatingthickness according to one embodiment of the present disclosure.

FIG. 1B is an illustration of a configuration comprising components ofsystem 100 according to one embodiment of the present disclosure.

FIG. 2 is a flow diagram of a method for measuring thin film/coatingthickness according to one embodiment of the present disclosure.

DETAILED DESCRIPTION

Embodiments according to this disclosure provide methods and systems forinline real-time measurement of thin film or coating thickness, and moreparticularly to methods and systems for inline real-time measurement ofthin film or coating thicknesses, including films or coatings of lessthan 0.2 microns, on a moving substrate.

Illustrative Embodiment

In one illustrative embodiment, a manufacturer employs the systems andmethods of the present invention to measure the thickness of alubricious coating applied to thin metal sheeting to ensure a sufficientlayer of lubricious coating is present on the thin metal sheeting—usedby the manufacturer to create its products—to prevent damage toexpensive manufacturing equipment that processes the metal sheeting. Inthe illustrative embodiment, the manufacturer incorporates a broadbandlight source that directs light waves through polarizers and wave platesonto the surface of the thin metal sheeting containing the lubriciouscoating as it is moving through the equipment. The manufacturer furtherincorporates detectors that capture reflected light that passes throughrotating analyzers. A computer controlling the manufacturing process isin communication with the light sources and the detectors and isprogrammed to configure the light sources to generate light within aparticular spectrum range based on the particular metal and theparticular lubricious coating being measured. The computer is furtherprogrammed to receive light wave data from the detectors. The computerquantifies the phase shift and polarization state changes of thereflected light, compared to the light waves generated by the lightsource, and then uses that information to evaluate and validatethickness of the lubricious coating at various locations on the metalsheeting as it is moving through the equipment.

In addition, rotating analyzers and detectors are coupled to anadjustment mechanism in communication with the computer. The computeroperates to detect flutters, vibrations, or other disturbances in themovement of the metal sheeting and automatically adjusts the positionsof the analyzers and detectors to ensure accuracy of the thicknessmeasurements.

In the illustrative embodiment, a preferred thickness range, warningthickness level and a critical thickness threshold are defined andprogrammed into the computer. If the measured thickness of thelubricious coating is well within the preferred thickness range, thecomputer allows the manufacturing process to continue. If the computerdetects that the lubricious coating is outside of the preferredthickness range, the computer provides feedback to the systemcontrolling the application of the lubricious coating. In response, thatsystem adjusts the application of the lubricious coating to bring itback within the preferred thickness range. In the event that the coatingthickness reaches the warning thickness level, the computer alerts theequipment operators of a potential malfunction in the lubricious coatingapplication system. The operators may then choose whether to shut downthe manufacturing process to investigate or to continue the process.

Illustrative System

FIG. 1A is a block diagram of a system for measuring thin film/coatingthickness according to one embodiment of the present disclosure. FIG. 1Aillustrates a configuration comprising components of system 100according to one embodiment of the present disclosure. System 100 maycomprise one of a variety of form factors. In one embodiment, system 100may be a self-contained system comprising a single housing. In oneembodiment, the system 100 may comprise a portable housing. In otherembodiments, the system 100 may be integrated directly intomanufacturing or testing equipment. In still another embodiment, system100 may comprise a number of components in separate physical locations,but coupled through wired and/or wireless communications and/ornetworking well known to those having ordinary skill in the art.

Embodiments of the present disclosure can be implemented in combinationwith, or may comprise combinations of: digital electronic circuitry,computer hardware, firmware, software, light sources, optical equipment,and/or optical sensors. The system 100 shown in FIG. 1 comprises aprocessor 102. The processor 102 receives input signals and generatessignals for communication, display, and processing sensor readings tomeasure thicknesses of thin films/coatings. The processor 102 includesor is in communication with one or more computer-readable media, such asmemory 104, which may comprise random access memory (RAM).

The processor 102 executes computer-executable program instructionsstored in memory 104, such as executing one or more computer programsfor providing a user interface and/or processing sensor readings tomeasure thicknesses of thin films/coatings. Processor 102 may comprise amicroprocessor, a digital signal processor (DSP), anapplication-specific integrated circuit (ASIC), one or more fieldprogrammable gate arrays (FPGAs), or state machines. The processor mayfurther comprise a programmable electronic device such as a PLC, aprogrammable interrupt controller (PIC), a programmable logic device(PLD), a programmable read-only memory (PROM), an electronicallyprogrammable read-only memory (EPROM or EEPROM), or other similardevices.

Memory 104 comprises a computer-readable media that may storeinstructions, which, when executed by the processor 102, cause it toperform various steps, such as those described herein. Embodiments ofcomputer-readable media may comprise, but are not limited to, anelectronic, optical, magnetic, or other storage or transmission devicecapable of providing the processor 102 with computer-readableinstructions. Other examples of media comprise, but are not limited to,a floppy disk, CD-ROM, magnetic disk, memory chip, ROM, RAM, ASIC,configured processor, all optical media, all magnetic tape or othermagnetic media, or any other medium from which a computer processor canread. Also, various other devices may include computer-readable media,such as a router, private or public network, or other transmissiondevice. The processor 102, and the processing, described may be in oneor more structures, and may be dispersed through one or more structures.

Referring still to FIG. 1, the system 100 also comprises one or moreuser input devices 108 in communication with the processor 102. Forexample, in some embodiments a user input device 108 may comprise akeyboard, mouse, trackball, touchscreen, touchpad, voice recognitionsystem or any other input device known to one having ordinary skill inthe art.

The system 100 also comprises a display 106. Display 106 is incommunication with processor 102 and is configured to display outputfrom the processor 102 to the user. For instance, in one embodiment,display 106 is a standard computer monitor such as an LCD display or acathode ray tube (CRT). In another embodiment, system 100 may comprise atouch-screen LCD that operates both as a display 106 and a user inputdevice 108. Various sizes of LCD displays may be used.

Referring now to FIGS. 1A and 1B, the system 100 further comprises alight source 110. For example, in one embodiment light source 110 is abroadband light source capable of generating light waves of multiplewavelengths. For example, the wavelengths of the illuminated light couldbe in the ultraviolet (UV), visible, or near infrared (NIR) regions. Inone embodiment, light source 110 is capable of generating light waveshaving wavelengths in the UV and visible spectrum regions. In one suchembodiment, the light source 110 comprises a xenon arc lamp. In anotherembodiment, light source 110 is capable of generating light waves havingwavelengths in the visible and NIR spectrum regions. In one suchembodiment, the light source 110 comprises a tungsten halogen lamp. Instill another embodiment, light source 110 is capable of generatinglight waves having wavelengths in the UV, visible, and NIR spectrumregions. In one such embodiment, the light source 110 comprises a xenonarc lamp and a tungsten halogen lamp.

In one embodiment, light source 110 is coupled to processor 102 to allowthe processor 102 to control the output of the light source 110. Forexample, the processor 110 may communicate with the light source 110 toturn the light source 110 on or off, or to specify the type of lightwaves to be provided. In one embodiment, the processor communicates withthe light source to specify light waves within the UV, visible, and/orNIR spectrum regions, or subsets thereof. In one embodiment, theprocessor 102 controls the amount of light generated to ensure the lightlevels are not saturated. In one embodiment, the light source 110 may bepositioned to directly emit light towards a substrate 120, as shown inFIG. 1B. In other embodiments, the light guides may be used to directlight emitted from a light source 110 located at another position, suchas within a housing comprising the processor 102 and memory 104, towardsa substrate 120.

The system 100 further comprises a polarizer 118. Polarizer 118 is anoptical device that functions to convert unpolarized light waves passingthrough it, such as light waves provided by light source 110, intolinearly polarized light waves. In one embodiment, a Glan Taylorpolarizer with an extinction coefficient of 10⁵:1 is used to convert thenon-polarized light beam into linearly polarized light beam. In theembodiment illustrated in FIG. 1B, polarizer is positioned such thatlight waves provided by light source 110 pass through polarizer 118.

The system 100 further comprises a wave plate 112. In one embodiment,the wave plate 112 is a quarter-wave plate Wave plate 112 functions toalter the polarization state of light waves passing through it. Forexample, a quarter-wave plate converts linearly polarized light wavespassing through it into circularly polarized light waves. In theembodiment illustrated in FIG. 1B, the wave plate 112 is positioned toreceive light waves from a light source 110 that first passes through apolarizer 118. In the embodiment illustrated in FIG. 1B, the light wavespassing through the wave plate 112 are incident light waves to asubstrate 120 comprising a coating or film 122 on the top surface. Insome embodiments, the wave plate 112 is rotatable and comprises amechanism (e.g. an electric motor) for rotating the wave plate 112. Inone such embodiment, processor 102 communicates with wave plate 112 tocontrol whether and at what speed the wave plate 112 is rotating.

The system 100 further comprises an analyzer 114. In one embodiment,analyzer 114 is a rotating analyzer that receives elliptically polarizedlight reflected by substrate 120 and/or coating or film 122. Therotating analyzer 114 functions to reflect light from various angularpositions. The analyzer 114 is the same component as the polarizer 118except that it is used to analyze the polarization state of the lightwave instead of altering the polarization state of the incident lightbeam. In some embodiments, a rotating analyzer 114 comprises a mechanism(e.g. an electric motor) for rotating the analyzer 114. In one suchembodiment, processor 102 communicates with the rotating analyzer 114 tocontrol whether and at what speed the analyzer 114 is rotating.

The system 100 further comprises a detector 116. Detector 116 operatesto detect the reflected light generated from various angular positionsof the rotating analyzer. In one embodiment, the detector 116 comprisesa spectrometer. In some embodiments, different detectors may be used fordifferent wavelength ranges. In one embodiment, the detector 116 may bepositioned to directly receive the reflected light generated fromvarious angular positions of the rotating analyzer 114, as shown in FIG.1B. In other embodiments, a probe connected to a light guide may be usedto capture and direct the reflected light generated from various angularpositions of the rotating analyzer 114 to a detector 116 located atanother position, such as within a housing comprising the processor 102and memory 104. In one such embodiment, the probe comprises a fiberoptic probe. The detector 116 operates to convert captured light wavesinto light wave data. In one embodiment, light wave data may be avoltage signal waveform that corresponds to the captured light wave. Inanother embodiment, light wave data comprises a data structurecontaining information that describes the capture light waves.

The detector 116 is in communication with the processor 102 and providesthe light wave data to the processor 102. The processor 102 isprogrammed to validate and evaluate the light wave data. In oneembodiment, the processor 102 quantifies the phase shift andpolarization state changes, compared to the light waves generated by thelight source 110, and then uses that information to evaluate andvalidate thickness of the coating/film 122 on the substrate 120. In oneembodiment, the processor 102 broadly calculates the polarization statechange and the phase shift of the incident light waves on the sample tothat of the reflected light waves emanating from the analyzer 114. Inone embodiment, theoretical models are developed for the given substratecoating combination and the Levenberg-Marquardt algorithm is used tocalculate the best fit to match the light wave data with a theoreticalmodel to determine thickness. In some embodiments, triangular smoothingtechniques are applied to light wave data to optimize the quality ofspectral response before it is evaluated. Furthermore, in someembodiments, techniques for determining signal quality and detectingnoise are used to validate light wave data corresponding to individualmeasurements. In one embodiment, signal quality of the light wave datais determined by using predetermined coating specific spectralsignatures to validate individual measurements. In still anotherembodiment, the processor 102 determines the strength and quality of thelight waves based on the light wave data and dynamically adjusts thelight intensity provided by light source 110.

In some embodiments, system 100 may comprise two or more sets of lightsources 110, polarizers 118, wave plates 113, analyzers 114 anddetectors 116. In one such embodiment, the system 100 may simultaneouslymeasure the thickness of the coating/film 122 at multiple locations onsubstrate 120. In another embodiment, a single light source 110 and/or asingle detector 116 may be used in conjunction with two or more sets ofpolarizers 118, wave plates 113, and analyzers 114. In one embodiment,an optical switch and light guides coupled thereto may be used toprovide light from a single light source to multiple locations onsubstrate 120. In another embodiment, an optical switch with lightguides attached thereto and probes coupled to the light guides may beused to capture the reflected light generated from various angularpositions of multiple rotating analyzers 114 and direct the capturelight to a single detector 116.

While shown as individual components in FIG. 1B, in some embodiments twoor more of light source 110 (or the emission point of a light guidecoupled to a light source 110), polarizer 118 and wave plate 112 mayreside in a single housing. Similarly, in other embodiments, analyzer114 and detector 116 (or a probe coupled to a detector 116 through alight guide) may reside in a single housing.

In one embodiment, analyzer 114 and detector 116 (or a probe coupled toa detector 116 through a light guide) are coupled to one or moreadjustment mechanisms 124 in communication with processor 102 foradjusting the position of the analyzer 114 and detector 116 (or a probecoupled to a detector 116 through a light guide). In another embodiment,polarizer 118 and analyzer 114 are coupled to one or more adjustmentmechanisms 124 in communication with processor 104 for adjusting theposition of the polarizer 118 and analyzer 114. In another embodiment,polarizer 118, analyzer 114, and detector 116 are coupled to one or moreadjustment mechanisms 124 in communication with processor 102 foradjusting the position of the polarizer 118 and analyzer 114. The one ormore adjustment mechanisms 124 may use electric motors, linearactuators, sliding tracks, gimbal mechanisms, or any other componentsknown to one having ordinary skill in the art.

Substrates and Coatings/Films

The present disclosure contemplates using the disclosed systems andmethods to measure the thickness of a wide variety of coatings/films 122on a wide variety of substrates 120. Contemplated substrates compriseall manner of metals (e.g. aluminum, copper, nickel, titanium, steel,tin plate and other metals employed as components of or in thefabrication of products or processing of materials), a variety of films(e.g. thin stretched films, thin coatings on PET film substrates,Polyethylene Film Substrates, etc.), glass, plastics, rubber, latex,silicon (e.g. circuit boards, wafers), and solar cells. Contemplatedcoatings comprise all manner of lubricants, waxes, liquids (e.g. water),silicone, thin films, UV coatings, nanometric coatings, adhesives, coldand hot end glass container spray coatings, printed electronics,anti-reflective (AR) coatings, CdTe coatings, and CdS coatings. Thepresent disclosure contemplates all methods for applying such coatingsto such substrates known to one of ordinary skill in the art. Forexample, some coatings may be sprayed onto a substrate. Other coatingsmay be rolled onto the substrate.

Operation of an Illustrative System

FIG. 2 shows a flow diagram illustrating the operation of a systemaccording to one embodiment of the present disclosure. In particular,FIG. 2 shows steps performed by a system to perform inline real-timemeasurement of thin film thickness on a moving substrate. To aid inunderstanding how each of the steps may be performed, the followingdescription is provided in the context of the illustrative diagrams ofembodiments of the system shown in FIGS. 1A and 1B. However, embodimentsaccording to the present disclosure may be implemented in alternativeembodiments.

Beginning at step 202, light waves are generated and directed toward acoated substrate 120. For example, the processor 102 communicates withthe light source 110 to generate light waves within a one or moreparticular spectrum ranges to be directed at a coated substrate 120. Inone embodiment, a user interface provided by the processor 102 anddisplayed on display 106 permits a system operator to identify thesubstrate and/or coating material and the processor 102 then determinesthe appropriate light waves to select based on materials informationstored in memory 104. In another embodiment, the processor 102 isprogrammed to take into account the traits of the substrate and/orcoating materials specified by a system operator to select theappropriate region of the wavelength spectrum for analysis. In stillanother embodiment, the user interface permits a system operator tomanually configure the wavelength range for analysis to be used for themeasurement process.

At step 203, the light waves provided by the light source 110 arelinearly polarized by passing through a polarizer 118. In addition, thelinearly polarized light waves are converted to circularly polarizedlight waves by passing through wave plate 112. In one embodiment, waveplate 112 is a rotating quarter-wave plate. In one such embodiment, theprocessor 102 communicates with the wave plate 112 to configure thespeed of the rotation of the rotating wave plate 112.

At step 204, elliptically polarized light waves reflected by thefilm/coating 122 and/or the substrate 120 are analyzed by an analyzer114. In one embodiment, the analyzer 114 is a rotating analyzer. In onesuch embodiment, the processor 102 communicates with the analyzer 114 toconfigure the speed of the rotation of the rotating analyzer 114.

At step 206, the detector 116 captures light waves reflected by the bythe coated substrate 120 and then further reflected from various angularpositions by analyzer 114.

At step 208, the detector 116 operates to convert the captured lightwaves into light wave data for communication to processor 102. In oneembodiment, light wave data may be a voltage signal waveform thatcorresponds to the captured light wave. In another embodiment, lightwave data comprises a data structure containing information thatdescribes the captured light waves. Once the light wave data isgenerated, it is communicated to the processor 102.

At step 210, the light wave data is processed by processor 102 todetermine the thickness of the film/coating 122 on the surface ofsubstrate 120. In one embodiment, processor 102 quantifies the phaseshift and polarization state changes, compared to the light wavesprovided by the light source 110, and then uses that information toevaluate and validate thickness of the coating/film 122 on the substrate120 using techniques disclosed herein. In other embodiments, theprocessor 102 may process the light wave data to determine other opticalparameters of the film/coating 122 such as refractive index, surfaceroughness, and extinction coefficient.

At decision point 212, it is determined whether the light wave data wasvalid. In one embodiment, the system determines whether there arevibrations, fluttering, or other disturbances in the movement of amoving substrate 120 that require adjustment of system components toobtain accurate measurements. If present, disturbances such asvibrations and flutter may impact the plane of incidence and thereflection of the light waves. In another embodiment, the systemdetermines whether the light source 110 is providing too little or toomuch light. In one embodiment, disturbances and/or light level defectsare identified based on the light wave data detected by detector 116during the performance of previous iterations of the presently-describedmethod. In some embodiments, the quality of the light wave data isvalidated using one or more techniques disclosed herein.

If the light wave data is validated, indicating that there are novibrations, fluttering, or other disturbances in the movement ofsubstrate 120 and no light level defects that require adjustment thenthe method proceeds to step 202 to perform another iteration of themethod. However, if the validation process determines that the data isnot valid, the measurement based on the light wave is discarded and themethod proceeds to step 214.

At step 214, adjustments are made to compensate for detecteddisturbances and/or light level defects. In one embodiment, theorientation and/or position of the optics such as analyzer 114 andpolarizer 118 are adjusted in real-time to accommodate for any changesin the plane of incidence. In another embodiment, the orientation and/orposition of the analyzer 114 and detector 116 are adjusted in real-timeto accommodate for any changes in the plane of incidence. In anotherembodiment, the orientation and/or position of polarizer 118, analyzer114, and/or detector 116 are adjusted in real-time to accommodate forany changes in the plane of incidence. In one embodiment, processor 102communicates commands to one or more adjustment mechanisms 124 to causeadjustment of the polarizer 118, analyzer 114, and/or the detector 116based on the detected vibrations, fluttering, or other disturbances inthe movement of substrate 120. In another embodiment, the processor 102communicates with light source 110 to adjust the intensity of the light.

General

The foregoing description of some embodiments of the disclosure has beenpresented only for the purpose of illustration and description and isnot intended to be exhaustive or to limit the invention to the preciseforms disclosed. Numerous modifications and adaptations thereof will beapparent to those skilled in the art without departing from the spiritand scope of the invention.

Reference herein to “one embodiment” or “an embodiment” means that aparticular feature, structure, operation, or other characteristicdescribed in connection with the embodiment may be included in at leastone implementation of the invention. The invention is not restricted tothe particular embodiments described as such. The appearance of thephrase “in one embodiment” or “in an embodiment” in various places inthe specification does not necessarily refer to the same embodiment. Anyparticular feature, structure, operation, or other characteristicdescribed in this specification in relation to “one embodiment” may becombined with other features, structures, operations, or othercharacteristics described in respect of any other embodiment.

We claim:
 1. A system comprising: a processor; a moving substratecomprising a coating applied by a coating applicator; a light source incommunication with the processor, the light source configured to provideand direct light waves of varying wavelengths toward the movingsubstrate; a polarizer positioned between the light source and themoving substrate; a wave plate positioned between the polarizer and themoving substrate; an analyzer positioned to receive light wavesreflected by the moving substrate; a detector, in communication with theprocessor, positioned to capture light waves reflected by the analyzerand configured to generate light wave data based on the captured lightwaves; and a memory in communication with the processor, wherein thememory comprises computer program code executable by the processor todetermine a thickness of the coating based on the light wave data, andwherein the memory further comprises computer program code executable bythe processor to, in response to a determination that the thickness ofthe coating is outside a predetermined range, generate a signalinstructing the coating applicator to adjust the rate of application ofthe coating onto the moving substrate.
 2. The system of claim 1, whereinthe coating is less than 0.2 microns thick.
 3. The system of claim 1,wherein the coating is a lubricious coating.
 4. The system of claim 1,wherein the coating is a liquid coating.
 5. The system of claim 1,wherein the wave plate is a rotating quarter-wave plate.
 6. The systemof claim 1, wherein the analyzer is a rotating analyzer in communicationwith the processor.
 7. The system of claim 1, wherein the computerprogram code executable by the processor to determine the thickness ofthe coating based on the light wave data comprises computer program codeexecutable by the processor to determine phase shift and polarizationstate changes of captured light waves compared to the provided lightwaves based on the light wave data.
 8. The system of claim 1, whereinthe computer program code executable by the processor to determine thethickness of the coating based on the light wave data comprises computerprogram code executable by the processor to apply theLevenberg-Marquardt algorithm to match the light wave data with atheoretical model.
 9. The system of claim 1, wherein a position and anorientation of the analyzer are adjustable and a position and anorientation of the detector are adjustable.
 10. The system of claim 9,wherein the memory further comprises computer program code executable bythe processor to: detect a disturbance in the movement of the movingsubstrate based on the light wave data; and adjust the position or theorientation of the analyzer or adjust the position or the orientation ofthe detector based on the detected disturbance.
 11. The system of claim9, wherein the memory further comprises computer program code executableby the processor to: detect a disturbance in the movement of the movingsubstrate based on the light wave data; and adjust the position or theorientation of the analyzer and adjust the position or the orientationof the detector based on the detected disturbance.
 12. The system ofclaim 1, wherein the memory further comprises computer program codeexecutable by the processor to adjust the intensity of the providedlight waves based on the light wave data.
 13. The system of claim 1,wherein the signal instructs the coating applicator to decrease the rateof application of the coating onto the moving substrate.
 14. The systemof claim 1, wherein the signal instructs the coating applicator toincrease the rate of application of the coating onto the movingsubstrate.